1. Field of the Invention
The present invention relates to a system of irradiating an object with a radiation ray to pick up a rear-projected image and of three-dimensionally presenting the picked-up image.
2. Description of the Related Art
Irradiating an object with X-rays to pick up rear-projected images is common in, for example, medical diagnosis. However, since the projected images are picked up, the information concerning all the regions which the radiation rays pass through is superimposed and, therefore, it is difficult to determine the distribution of diseased parts on a three-dimensional space.
Technologies in which an object is irradiated with X-rays in all directions to pick up rear-projected images, the data is reconstructed to acquire voxel data, and tomographic images on an arbitrary slice plane are displayed by rendering have also been developed. With such technologies, it is possible to easily determine three-dimensional positions of diseased parts. However, there are problems in that artifact contamination is caused in the reconstruction of the data and that the resolution of the reconstructed images becomes lower than that of the picked-up images.
One technique for resolving such problems is so-called “stereoscopic observation”. The stereoscopic observation, in which rear-projected images are directly observed, has the advantages of observing high-resolution images and of easily determining the stereoscopic depth of diseased parts by stereoscopic vision using parallax.
A system of presenting stereoscopic images, applicable to the presentation of images in the stereoscopic observation, is disclosed in, for example, U.S. Pat. No. 6,256,372 (Aufrichtig, et al.) and U.S. Pat. No. 6,359,961 (Aufrichtig, et al.). In such a system of presenting stereoscopic images, a radiation source is moved on a surface having an arbitrary shape to pick up images (parallax images) from multiple viewpoints in time division multiplexing. Observing two arbitrary parallax images from the corresponding two viewpoints achieves observation of a stereoscopic image.
However, the difficulty in directly performing the stereoscopic observation of the images varies among individuals. Accordingly, it is preferable to use three-dimensional display devices that allow anyone to easily perform the stereoscopic observation. Medical diagnosis by the stereoscopic observation using such three-dimensional display devices is proposed in academic societies (refer to Natsuko Bandai, 2001, “Sanjigen-display wo mochiita atarashii kyoubu X-sen satsuei-ho”, Journal of Chubu division of Japanese Society of Radiological Technology Vol. 3 No. 1).
In a general digital photography field, technologies of generating arbitrary viewpoint images based on image-based rendering, such as ray interpolation, which technologies are capable of generating images of viewpoints arbitrarily selected, are known (U.S. Pat. No. 6,268,862 (Uchiyama, et al.) and Marc Levoy et al., 1996, “Light Field Rendering” Proceedings of Special Interest Group for Computer Graphics (SIGGRAPH)).
There are various presentation devices, such as three-dimensional displays and three-dimensional printers, which output stereoscopic images. In order to achieve a similar stereoscopic view in the various presentation devices, it is necessary to pick up rear-projected images by a trial-and-error method in consideration of the characteristics of the individual presentation devices.
For example, it is necessary to pick up rear-projected images in consideration of technical matters, including the difference in the number of viewpoints among three-dimensional displays, a variation in the ratio of the angle of view at the time of input to the angle of view at the time of output, the parallax limit within which the images can be presented, and the difference in the method of emitting light rays. Accordingly, when persons who have no technical knowledge about the three-dimensional displays take the images, the stereoscopic view of the stereoscopic images is varied, thus giving different impressions to an observer. In other words, in order to present natural stereoscopic views, it is necessary to pick up images of an object in consideration of the parameters of the three-dimensional displays and, also, of the parameters of the image pickup device.
Also in the application of the technologies of generating arbitrary viewpoint images to the three-dimensional display of rear-projected images, it is necessary to calculate multiple virtual radiation source positions in consideration of the display parameters of the three-dimensional display devices, a virtual viewpoint position, which is a candidate for the observation viewpoint, and the image pickup parameters of the image pickup device and to generate the rear-projected images corresponding to the virtual radiation source positions, as in the normal display of stereoscopic images. Hence, it is difficult to provide the stereoscopic images having expected stereoscopic views only by simply applying the above technologies.
In order to realize the generation of arbitrary viewpoint images, in which the virtual viewpoint positions can be freely varied within a predetermined range, storage of a group of original images, picked up while the radiation source is moved to multiple positions, is required as a preparatory work. However, since there is no guideline on how to select the positions of the radiation source, an excessive number of original images can be picked up. The pickup of the excessive number of original images causes problems in that the radiation dosage to the object is increased, it takes excessive time to pick up the images, and an excessive amount of storage or photographic material is consumed. In contrast, the number of the original images that is too small can cause a reduction in the quality of the stereoscopic images.